1. Field of the Invention
The present invention relates to a radiation boiler which serves in particular to recover the process heat produced during the gasification of ash-forming, carbonaceous fuels.
2. Discussion of the Prior Art
The utilization of this energy is absolutely essential for reasons of economy in all those cases in which the gas can be used without otherwise utilizing the sensible heat or enthalpy.
The production of steam with the aid of heat generated in the process is, however, as a rule particularly difficult if liquid ash particles are entrained in the gas phase, as is typically the case in certain gasification processes carried out under pressure, e.g. processes involving hard coal or ash-forming petroleum. Additional problems arise on account of the fact that the ash has, depending on the feedstock material, a varying composition and thus varying physical properties, which places particular requirements on the construction of a waste heat recovery plant.
German Offenlegungsschrift 27 05 558, corresponding to U.S. application Ser. No. 876,446 filed Feb. 9, 1978 and now abandoned, and Ser. No. 130,643 filed Mar. 17, 1980 and now U.S. Pat. No. 4,310,333 both of which are commonly assigned with this application and the disclosures of which are hereby incorporated herein by reference, describes a method for the gasification of solid fuels in which the reaction gas together with the combustion residues are cooled in a radiation boiler arranged directly beneath the reactor to such an extent that the liquid combustion residues, which are entrained by the gas as fine droplets, solidify before they reach a convection boiler connected downstream. The combustion residues occurring in the form of coarse agglomerates are precipitated in a water bath arranged in the lower part of the radiation boiler when the gas stream containing the residues is deflected at the surface of the said water bath.
In this known method the ash is largely precipitated out, the reaction mixture being pre-cooled. The temperature of the particles remaining in the gas is lowered to such an extent that sintering on the heat exchanger surfaces is prevented. Heat transfer predominantly occurs by means of radiation. Direct contact between the liquid ash and the heat exchanger wall is excluded since the diameter of the central space is chosen sufficiently large in relation to the inlet opening for the gas and the length of the heat exchanger elements. In a second cooling stage, connected to the radiation boiler, the heat transfer essentially takes place by means of convection. Although the known method for heat recovery has proved itself in practice, it is still capable of improvement with respect to certain aspects. Thus, although the degree or precipitation of the ash is more than 90% and is thus surprisingly high, a further improvement in the ash precipitation is desired. Moreover, heat transfer is prevented by a very loose, continuous layer of ash a few millimeters thick on the heat exchanger surfaces even when liquid ash does not reach the wall. Although this layer of ash comes away locally from the heat exchanger surfaces as soon as it reaches a certain thickness, it nevertheless causes a considerable reduction in the heat transfer.
As soon as the liquid ash droplets enter the water bath, solid ash having a relatively low bulk density is preferentially formed. In order to be able to keep the dimensions of the discharge locks from the pressure system as small as possible, an attempt is made to obtain coarse-grained and relatively dense ash. The formation of coarse-grained ash is also advantageous for the distribution of the total ash into low carbon content coarse ash and carbon-containing fine ash, as the fine ash is returned to the gasification process. Finally, it would also be noted that according to the known method the enthalpy of the liquid ash is almost completely lost.